1. Field of the Invention
The present invention generally relates to a method for producing a metal/ceramic bonding circuit board. More specifically, the invention relates to a method for producing a metal/ceramic bonding circuit board wherein a metal plate is bonded to a ceramic substrate via a brazing filler metal to form a circuit and on which a large power element, such as a power module, is suitably mounted.
2. Description of the Prior Art
In a typical method for producing a ceramic circuit board for a power module or for mounting a semiconductor thereon, a metal plate and a ceramic substrate are first bonded to each other. For example, there is industrially utilized the direct bonding method for arranging a copper plate on a ceramic substrate so as to allow the copper plate to directly contact the ceramic substrate and for heating the copper plate and the ceramic substrate in an inert gas to bond the ceramic substrate and the copper plate to each other. There is also industrially utilized the brazing and soldering method for arranging a copper plate on a ceramic substrate via a brazing filler metal containing an active metal, such as Ti, Zr or Hf, and for heating them in vacuo to bond the ceramic substrate and the copper plate to each other. In the brazing and soldering method, the active metal concerns the bonding of the ceramic substrate to the metal plate, and the ceramic substrate reacts with the brazing filler metal to form a reaction product. It is generally considered that a brazing filler metal reacts with a ceramic substrate of an oxide, such as Al2O3, to form an oxide of an active metal, reacts with a ceramic substrate of a non-oxide, such as AlN or Si3N4, to form a nitride of an active metal, and reacts with a ceramic substrate of a carbide, such as SiC, to form a carbide of an active metal, so that the product bonds the ceramic substrate to the copper plate. That is, the brazing filler metal layer after bonding comprises a layer mainly containing the metal of the brazing filler metal, and a layer mainly containing an interface product of the interface between the brazing filler metal and the ceramic substrate.
As a method for forming a predetermined circuit shape by patterning after bonding a metal plate, such as a copper plate, for a circuit or radiation, there is an etching method which is also utilized for a printed circuit board or the like. This method is widely utilized since it is easy to obtain a fine pattern and it is possible to relatively simply cope with the change of a circuit design. In this method, for example, a mixed solution of iron chloride or copper chloride, hydrochloric acid and hydrogen peroxide is usually used as an etchant for a metal plate, such as a copper plate. In the case of the above described direct bonding method, this etchant can carry out etching and patterning without causing problems since it is possible to ignore reaction products. However, in the case of the brazing and soldering method, this etchant can dissolve the metal plate, but it can not dissolve the brazing filler metal and the reaction product of the brazing filler metal with the ceramic substrate (the general term for the brazing filler metal and the reaction product will be hereinafter referred to as a “brazing filler metal and so forth”), so that the brazing filler metal and so forth remain between circuit patterns and/or on the edge face of the substrate. Since the brazing filler metal and so forth are conductors, it is not possible to satisfy basic characteristics of a circuit board, which isolate the circuit patterns from each other and/or the surf ace and reverse of the board from each other. As a method for removing the brazing filler metal and so forth, there is known a method for using hydrofluoric acid alone or a mixed acid of hydrofluoric acid and at least one inorganic acid selected from the group consisting of nitric acid, sulfuric acid and hydrochloric acid, or for using a solution containing aqua regia, sodium hydroxide and/or potassium hydroxide, to treat and remove the brazing filler metal and so forth (see Japanese Patent No. 2,594,475). There is also known a method for treating the brazing filler metal and so forth with a solution containing a hydrogen halide and/or an ammonium halide, and then, treating them with a solution containing an inorganic acid and hydrogen peroxide, to remove the brazing filler metal and so forth (see Japanese Publication No. 7-36467).
The sectional shape of the end portion of a circuit pattern of a metal/ceramic bonding circuit board, wherein a metal plate is bonded to a ceramic substrate via a brazing filler metal, greatly concerns characteristics of the circuit board, so that it is desired that the optimum shape of the end portion is formed in accordance with required characteristics. That is, in order to enhance reliability against thermal shocks, the protrusion of the brazing filler metal from the edge portion of a copper plate is effective in suppression of the thermal stress caused by the difference in coefficient of thermal expansion between the metal plate and the ceramic substrate bonded thereto.
It is known that the protrusion of the brazing filler metal is greatly effective in thermal shock resistance. As a conventional method for producing such a protrusion, a so-called twice-resist and twice-etching method or a so-called once-resist (removing) method is adopted when the brazing filler metal is removed. The twice-resist and twice-etching method is a method for applying a resist on a predetermined portion of the surface of a metal plate, etching undesired portions of the metal plate to form metal circuits, removing the resist, removing undesired portions of a brazing filler metal remaining between the metal circuits, applying a resist on the surface of the metal circuits again, and etching undesired portions of the metal plate again, i.e., a method for carrying out resist processes twice and etching processes twice. On the other hand, the once-resist (removing) method is a method for applying a resist on a predetermined portion of the surface of a metal plate after bonding of the metal plate, etching undesired portions of the metal plate to form metal circuits, peeling off and removing the resist, and removing undesired portions of a brazing filler metal remaining between the metal circuits with, e.g., fluoride chemicals. In the former, it is possible to obtain the large protrusion of the brazing filler metal, but the number of processes is large and costs are high. In the latter, the number of processes is the same as that in usual methods, but it is difficult to increase the protrusion of the brazing filler metal from the nature of the chemicals. In addition, there are some cases where it is not possible to ignore attacks on the ceramic substrate from the nature of the chemicals. Moreover, the top of the metal plate is also dissolved in the chemicals, which removes the brazing filler metal, to cause shear drops. Therefore, a discrepancy is caused in the mounting of a chip, such as a semiconductor chip, so that there are some cases where it is required to carry out a design change, such as the increase of the size.
However, in conventional methods, it is difficult to control the length of the brazing filler metal and so forth protruding the edge portion of the metal plate (which will be hereinafter referred to as a “brazing filler metal protruding length”. In particular, if the brazing filler metal protruding length is set to be long, the number of processes is larger than that in usual processes, so that production costs increase. In addition, a brazing filler metal removing agent causes shear drops from the top of the metal plate to cause dimensional discrepancy when a chip, such as a semiconductor chip, is mounted thereon.
In addition, if chemicals containing hydrogen halide or ammonium halide, typically hydrofluoric acid, a mixed acid thereof or ammonium fluoride, are used as the brazing filler metal removing agent, there are some cases where the ceramic substrate is also corroded simultaneously with the removal of the brazing filler metal and so forth, so that characteristics with respect to reliability as the ceramic circuit board are deteriorated. On the other hand, if the corrosion of the ceramic substrate is intended to be suppressed, there is some possibility that it is not possible to completely remove the brazing filler metal and so forth between circuit patterns, so that the brazing filler metal and so forth remain, thereby causing defectives. In addition, since the ratio of the etch rate of the brazing filler metal to that of the metal plate is small, it is difficult to greatly protrude the brazing filler metal (e.g., >30 μm). Moreover, since halogen compounds, such as hydrofluoric acid, are easily corroded, production facilities, waste solution treating facilities and waste solution treatments are comparatively high in cost, so that production costs for products have a tendency to increase.
On the other hand, if an alkaline mixed solution of a chelating agent (an alkaline chelate removing agent) is used as the brazing filler metal removing agent, damage to ceramic is relatively small, and the solubility of the brazing filler metal is good, but a generally used alkali peeling type etching resist, together with the brazing filler metal, is removed. For that reason, in order to produce amore reliable substrate by protruding the brazing filler metal from the metal circuit portion, an etching resist printing process and an etching process must be carried out again, so that production costs are higher than usual processes.
On the other hand, an organic solvent peeling type etching resist withstands alkali solutions. However, in order to remove the resist, an environmentally toxic organic solvent, such as trichloroethylene, must be used. However, environmental measures can be carried out by facilities.